[GribinIsaf]

I have been searching the forums and there is a discussion of ASHPs here from 2012/13.  I wondered if anyone had recent experience or knowledge now that (presumably) the technology has moved on.

(I have obviously also been reading this recent thread

We returned one of the enquiry forms that are being sent out by installers, prompted by the end of the current RHI grants in March, out of curiosity more than anything.  Having thought more about it we are now considering moving on to the next stage of having a full viability survey.

We live an ancient building that was renovated for residential use thirty years ago or so.  It has thick stone walls which provide useless insulation and would not be viable to clad inside or out.  A third of the roof has no insulation as it is pitched with no loft space.

Our current heating is provided by an LPG boiler and a wood-burning Rayburn.  They are both linked to the radiators and hot water storage tank and can work independently or in tandem.  We don't want to get rid of the Rayburn because although we do buy in some wood (not cheap) we do also source quite a lot of free wood from our own land and that of neighbours. The guy who visited from the installer said that they could do a system where the ASHP and the Rayburn worked together in the same way.

We are attracted by the carbon reduction and from what we understand so far about a constantly warm house.  We don't anticipate a major reduction in ongoing fuel costs although we are looking (again) about the feasibility of solar on our roof.

I would be interested to hear if anyone has made the change recently and also from anyone has thoughts on the technicalities of our setup.  Many thanks

[Backinwellies]

I'm no expert but I understood for air/ground source heat pumps to work the house needs to be 'sealed'  so is never going to work in an old stone house (which actually needs to breath!) Did the guy who visited not tell you this? 

[Womble]

No, not "sealed" as such, but yes, it does have to be energy efficient.

We looked into both air source and ground source for our old stone house and decided that it just wasn't going to work for us.

I'm not sure the technology has moved on all that much either - a heat pump is just a fridge assembled back to front, so it was already proven technology. What has happened of course is that as production volumes increased, prices came down a bit.

The key performance parameter for a heat pump is the "COP", or coefficient of performance. So if a heat pump has a COP of say 3, you'll get 3kW of heat out for every 1kW of electricity you put in. The key thing to figure out is what the COP is going to be like for your conditions year round, because the colder it gets outside, the harder it is to extract heat from that atmospheric air and 'pump' it inside to your radiators (thus the achieved COP drops). Beware of salesmen who just quote figures based on, for example 10degC outside temperature - that won't be reflected year round.

[in the hills]

Our neighbours live in a rented cottage built in part in the 1700's. The estate had air source heat pumps fitted to their cottage and several others about 18 months ago.


They have found that it is very expensive to run and the cottage is not very warm. When they have looked into it they have been told that it was not the best heating option for their property due to the fact that it is a system that works best in well insulated properties. It is not really feasible for them to improve the insulation of their old cottage very much and they are now a little worried about keeping warm this winter.


Other people on the estate have had the same experience and faced with huge bills.


Just the experiences of a few people I know that have had these systems fitted recently.

[Womble]

Just to add - the other thing that affects the COP of an ASHP is the output temperature to the house. If you have a large surface area for heat transfer, as you get with underfloor heating, you can reduce the temperature of the water going into the house, which increases the efficiency (COP).

However, if you're tying into an existing radiator system, that has a relatively low surface area for heat transfer, so to get the same output, you need a higher temperature coming from the ASHP to keep the room at the same temperature, and this lowers the efficiency (COP).

To quote this consultant's website:

The CoP will vary with each installation, but the lower the output temperature to the heat distribution system the higher the CoP will be. If an output temperature of 60°C is needed to heat radiators the CoP is likely to fall to level of only 2.0. If the heat distribution is to a well designed underfloor heating system that works well at an output temperature of 40°C then the CoP can rise to a level of 3 in the winter (and higher in the spring and autumn).

If a system is only achieving a COP of two, that means it's only twice as efficient as having standard electric heaters. Ouch! That's going to equal some pretty hefty electricity bills!     (BTW I found a report which contains good real-life data on ASHP performance if anybody wants to research further. I can't link to the file, but if you google "staffell - 2009 - review_of_domestic_heat_pump_cop", you should find it)


This is why, as an engineer, my eyebrows raised rather a lot when I heard about the Government's plans to install heat pumps everywhere to save the planet. In the right situation they're great, but in the wrong one, they're a disaster. I'd far rather we insulated houses better (though I haven't yet glued myself to the M25 to demand that).

[SallyintNorth]

You can insulate to exclude draughts from windows and doors, and line the roof / roof space to prevent heat escaping upwards.  Stone walls are *meant* to breathe.  Thick stone walls work like a heat sink.  Get them properly dry and warm, then aim to maintain the temperature.  Allowing the temperature to fluctuate is not so good - but the stone walls will help with that themselves, once they are properly warm and dry.  Like huge storage radiators!

We have an air source heat pump and a heat recovery system in our old stone farmhouse.   The walls are not insulated but we fitted new triple glazed windows and all the doors are draughtproof.  The ground floor is communal space - a bathroom, a larder, large kitchen, large dining room - and there are two flats above.  The flat in the roof was recently converted and is well-insulated, the middle flat is older but both ceilings/floors are fire-proofed and heat- and noise-insulated.  All levels of the house can be kept nicely comfortable with the air source unless it gets very cold - which for us in North Cornwall means below 5C, and / or blowing a hoolie with rain or sleet from the north or east.  The dining room and the middle flat have wood burners which are used when needed, which mostly they are not.  The top flat has underfloor and tops up with an electric heater if needed, which is not very often - and she is Swiss and likes it *very* hot, like 21C+. 

Of course we cook in our communal kitchen maybe 4 or 5 times a week, and all that heat gets recovered by the heat recovery system, so that helps too.

Big thick radiators for heat pumps.  (Same for ground source, which we have in 6 of the houses.)  IMO, it works best having the water temp much as it comes out of the system, and maintaining a low constant heat so that the spaces don't go hot - cold - hot.  If sitting down for long periods, wear winter woollies, use a hot water bottle and a shawl if you need, rather than heating the entire space to 20C!  And go take the dog for a walk or chop some wood or something if you get cold; the house will feel toasty when you get back in! 

[GribinIsaf]

Many thanks to all for thoughts so far.  Some useful viewpoints clearly expressed

[GribinIsaf]

Looking at the info on the CAT website they talk about a heat exchanger using a bore hole.  Does anyone know about this...?  Can you use an existing bore hole...?

[chrismahon]

Tying into an existing central heating system won't work because the emitters are 'radiators' and won't provide the required heat output at reduced flow temperatures. You would need to change them all to 'convectors', which we have here. Hadn't seen them in the UK, but here they are the norm consisting of aluminium finned elements joined together to make the size, so output required. The flow arrives top right and exits bottom left, or vice versa. The flow temperature of ours are about 40C, as opposed to the 80C of radiators.


We have all stone walls and solid floors, but plenty of insulation in the loft.

[SallyintNorth]

Looking at the info on the CAT website they talk about a heat exchanger using a bore hole.  Does anyone know about this...?  Can you use an existing bore hole...?

As I understand it, it's a way of doing ground source without needing to dig up acres and acres for slinkies.  If you were putting in a borehole, I would think one would do it.  I don't know if it can be retrofitted - if you find out, please update the thread! 

[Dan]

Looking at the info on the CAT website they talk about a heat exchanger using a bore hole.  Does anyone know about this...?  Can you use an existing bore hole...?

We've got a GSHP with a horizontal ground loop, about 4 feet down. Our water table over winter is often at or above 3 feet, and we were told this is a good thing, because water is better for heat exchange than soil. We've been really happy with it anyway.

I'd imagine a bore hole would work well on that basis, and can't see why you couldn't use an existing hole provided it has sufficient depth. For reference our GSHP is 9kW, and the loop is about 400m. I think one of the factors with bore holes is adequate separation of the loop pipes so they're not competing, so you may need more than one bore hole.

[GribinIsaf]

Thanks for that Dan

We have decided not to proceed with the idea of an ASHP or GSHP.  One of the factors is that our current system has a wood burning Rayburn and an LPG boiler which can both power the central heating, either in tandem or individually.  We would not want to give up the Rayburn option as we do have some supply of free wood (although we also buy some in).  The water temperatures of the Rayburn would not be compatible with an ASHP system.